P-glycoprotein, an adenosine triphosphate (ATP)-dependent drug efflux pump, is overexpressed in multidrug-resistant (MDR) tumor cells. It reduces the intracellular concentration of cytotoxic xenobiotics, thereby decreasing the effectiveness of many cancer chemotherapeutic regimens. P-glycoprotein belongs to the ABC (ATP-binding cassette) superfamily of active transporters, and is encoded by a multigene family in higher eukaryotes. Mammalian P-glycoprotein family members can be divided into three classes. Class I and class II P-glycoproteins confer multidrug resistance whereas class III proteins do not.
In humans, P-glycoprotein is encoded by two linked genes (“MDR1” and “MDR3”) on chromosome 7q21.1. MDR3 functions as a lipid translocase and mutations in this gene are associated with familial intrahepatic cholestasis. MDR1 confers drug resistance on certain cancer cells. In addition to being overexpressed in cancer cells, MDR1 P-glycoprotein is widely expressed in normal, predominantly secretory and absorptive human tissues, where it functions in diverse physiologic processes including cellular differentiation, cell proliferation and cell survival. In these normal cell types, P-glycoprotein functions in the transmembrane release or uptake of xenobiotics and certain therapeutic drugs, small peptide molecules, certain steroid compounds, and phospholipids.
P-glycoprotein is also expressed by lymphoid cell populations from human bone marrow and the peripheral blood. Specifically, P-glycoprotein has been shown to be expressed on the membrane of pluripotent stem cells, monocytes, dendritic cells, CD4+ and CD8+ T lymphocytes, natural killer cells, and B lymphocytes. In immune cells, P-glycoprotein functions in the transport of cytokines and other small molecules, which are critical for physiologic immune responses to occur. Specific blockade of P-glycoprotein can suppress the immune response to alloantigen and nominal antigen. However, a degree of redundancy exists for P-glycoprotein function in these cell types, pointing to the existence of additional, hitherto unidentified related molecules.
Pluripotent stem cells and other tissue progenitor cells also possess a unique P-glycoprotein-like activity, characterized by decreased intracellular accumulation of fluorescent dyes, which allows for the specific isolation of these cell types for therapeutic uses. However, it is thought that this function is not mediated by MDR1 P-glycoprotein, but rather by a related, as yet unidentified, P-glycoprotein family member.
Despite the irrefutable role of MDR1 P-glycoprotein in cancer multidrug resistance, attempts to improve chemotherapy by inhibiting this protein have met with only limited success. Thus, it may be inferred that there are homologous proteins that, like MDR1, are able to make cells resistant to therapeutic agents. In addition, it may be inferred that MDR1 homologous proteins serve P-glycoprotein-like functions in physiologic human tissues, in particular in cells of the immune system, pluripotent stem cells and tissue progenitor cells, where either redundancy exists for MDR1 P-glycoprotein function, or where MDR1 P-glycoprotein is known to not promote the observed P-glycoprotein-associated activity.